The present invention relates to a magnetic recording medium such as a magnetic recording tape, etc., and particularly, the invention relates to a magnetic recording particulate medium comprising a support provided thereon a magnetic layer comprising a ferromagnetic powder dispersed in a binder wherein the magnetic layer is formed by coating a magnetic coating material containing a ferromagnetic powder and a binder on a support, and also relates to a magnetic recording medium excellent in the sensitivity and S/N in a short wavelength region. Furthermore, the invention relates to a magnetic recording particulate medium, which is particularly suitably used for the system of using an MR head utilizing a magnetic resistance effect for the playback (or reproduction).
Because magnetic recording media have the excellent features that the medium can be repeatedly used, the electronization of signal is easy and the formation of a system by the combination with peripheral instruments is possible, signals can be easily corrected, etc., the magnetic recoding media have been widely utilized in various fields such as video tapes, audio tapes, computer tapes, etc.
Also, to cope with the requirements such as small-sizing of instrument, the improvement of the quality of recording and reproducing signals, long-time recording, the increase of the recording capacity, etc., in regard to the recording media, further improvements in the recording density, the reliability, and the durability has always been desired.
For example, to cope with the realization of a digital recording system of realizing the improvement of the audio quality and the image quality and the development of image recording system corresponding to high-vision TV, a magnetic recording medium, which can record and reproduce shorter wavelength signals than conventional systems and is excellent in the reliability and the durability even when the relative speed of the medium and head is increased, has been required. Also, the development of a digital recording medium of a large capacity for storing increasing data for the use of computers has been desired.
For high-density recording of a magnetic recording (particulate) medium, various methods have been investigated and proposed for improving the magnetic characteristics of the magnetic layer owing to improvements in magnetic substance, for example, by using an iron or iron-base alloy magnetic powder in place of a magnetic iron oxide powder, which has hitherto been used, or making magnetic powder more fine, or owing to improvements in the packing property and the orienting property of magnetic powder, for improving the dispersibility of ferromagnetic powder, and for increasing the surface property of the magnetic layer.
For example, methods of using a ferromagnetic metal powder or hexagonal ferrites as a ferromagnetic powder for increasing the magnetic characteristics are disclosed, for example, in JP-A-58-122623 (the term xe2x80x9cJP-Axe2x80x9d as used herein means xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-61-74137, JP-B-62-49656 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means xe2x80x9cexamined Japanese patent publicationxe2x80x9d), JP-B-60-50323 and U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198.
Also, for increasing the dispersibility of a ferromagnetic powder, it is proposed to use various kinds of surface active agents (disclosed, for example, in JP-A-52-156606, JP-A-53-15803 and JP-A-53-1161114) and various kinds of reactive coupling agents (disclosed, for example, in JP-A-49-59608, JP-A-56-58135 and JP-B-62-28489).
Furthermore, for improving the surface property of the magnetic layer, a method of improving a surface-formation treatment after coating and drying is proposed as disclosed, for example, in JP-B-60-44725
For attaining a high recording density of a magnetic recording medium, it has strongly proceeded to shorten the wavelength of signal used. Because when the wavelength of the region of recording signal becomes the size comparable with the size of the magnetic powder used, a clear magnetizing transition state cannot be made, recording substantially becomes impossible. Accordingly, it is necessary to develop a magnetic powder having a particle size sufficiently smaller than the shortest wavelength used, and fining of the particle sizes of the magnetic powder has been intended for a long period of time.
In a metal powder for magnetic recording, by making the particle acicular to enhance shape anisotropy, the desired coercive force is obtained. It is well known for persons skilled in the art that it is necessary for high-density recording to make the particle sizes of a ferromagnetic metal powder fine and reduce the surface roughness of the magnetic recording medium obtained. However, in the metal powder for magnetic recording, with reducing the particle sizes, the acicular ratio is lowered and the desired coercive force cannot be obtained. Also, various investigations about the oxide layers formed on the surfaces of ferromagnetic metal powders for magnetic recording have been made by various methods, and the estimated values about the oxide layer thickness, the structure of oxides, and the magnetization of oxides are reported. Specifically, it is reported that the oxide composition is a spinel-type iron oxide (FeOx:1.33xe2x89xa6xxe2x89xa61.5), and the saturation magnetization of the iron oxide is about 40 Axc2x7m2/kg.
Recently, for a data recording system for computers, a reproducing head (MR head) having a high sensitivity utilizing a magnetic resistance effect has been used, and the system noise is governed by the noise originated in a magnetic recording medium. Also, in the MR head, a reduction of the noise generation due to static electricity is one of problems.
That is, the development of a magnetic recording medium, which is in conformity with the characteristics of a magnetic resistance-type head (MR head) and further shows good electromagnetic characteristics, has been desired. Also, a magnetic recording medium, which is suitably used for a magnetic recording reproducing system combined with the MR magnetic head capable of recording at a high data transferring speed and at a high density, has been required.
The present invention has been made in view of the above-described problems of the related arts, and an object of the invention is to provide a ferromagnetic metal powder capable of producing a magnetic recording medium giving a short wavelength output and having a good S/N and a magnetic recording medium containing the ferromagnetic metal powder.
As the result of various investigations on the production method of a fine particulate ferromagnetic metal powder, a ferromagnetic metal powder containing MAl2O4 (wherein M is a transition metal) in the oxide of the surface layer thereof. Also, it has been found that because in the ferromagnetic metal powder, a non-magnetic sintering inhibitor and MAl2O4 existing on the surface of the particles thereof scarcely have magnetization, the interaction between the particles becomes small and when the ferromagnetic metal powder is highly packed in the magnetic layer of the magnetic recording medium, the coercive force is not lowered, and owing to the small interaction between the particles, the noise level of the magnetic recording medium is lowered. Also, it has been found that even by the ferromagnetic metal powder having small particle sizes, by which it has hitherto been difficult to obtain a high coercive force, the acicular ratio of the central metal portion can be increased and owing to the small interaction between the particles, the ferromagnetic metal powder having a high coercive force is obtained. Thus, the present invention has been accomplished.
That is, the present invention provides a ferromagnetic metal powder for magnetic recording, containing iron as the main constituent and containing MAl2O4 (wherein, M is a transition metal), wherein the coercive force (Hc) is from 135 to 240 kA/m, the saturation magnetization ("sgr"S) is from 100 to 155 Axc2x7m2/kg, the average major axis length of the particles is from 30 to 80 nm, the average acicular ratio of the particles is from 4.0 to 8.0, and the variation coefficient of the major axis length of the particles is from 3 to 25%.
Also, the invention provides a magnetic recording medium comprising a support having provided thereon a non-magnetic layer mainly containing a non-magnetic powder and a binder and at least one magnetic layer provided on the non-magnetic layer, wherein the magnetic layer contains the above-described ferromagnetic metal powder.